The present invention relates to a fluoran color forming dye precursor which alone can form different colors by controlling the heat applied, and a color forming recording material using the same.
Recording materials which utilize color forming reaction of an electron donating dye precursor (hereinafter referred to as "dye precursor") and an electron accepting color developer (hereinafter referred to as "color developer") are well-known as pressure-sensitive recording materials, heat-sensitive recording materials, reversible heat discoloration materials and others.
For example, heat-sensitive recording materials comprise a support and a heat-sensitive recording layer mainly composed of a dye precursor and a color developer provided on the support, and the dye precursor and the color developer instantaneously react with each other upon heating with a thermal head, a thermal pen, a laser beam, etc. to produce recorded images. These are disclosed in JP-B-43-4160, JP-B-45-14039, etc. Since these heat-sensitive recording materials are relatively inexpensive and recording devices are compact and are relatively easy in maintenance, they are used in a wide variety of the fields of not only recording media for facsimile and various computers, but also labels for terminal devices of printers and computers, and automatic vending machines for tickets.
However, with extension of uses of heat-sensitive recording materials, performances and quality required are diversified, and enhancement of sensitivity, increase in fastness of images, reversible recording and multicolor recording are being investigated. Especially, the multicolor recording is wide in the scope of application and a number of recording materials used therefor have been proposed.
Among conventional multicolor recording materials, in the case of two-color heat-sensitive recording materials, colors different at the time of low-temperature heating and high-temperature heating are formed by controlling the heat energy, and the method for multicolor formation can be roughly classified into two methods. According to the first method, color formation of the low-temperature heat-sensitive color forming layer is effected at the time of low-temperature heating and color formation of both the low-temperature heat-sensitive layer and the high-temperature heat-sensitive layer is effected at the time of high-temperature heating to obtain a mixed color. According to the second method, color formation of only the high-temperature heat-sensitive layer is effected at the time of the high-temperature heating by using a color eraser having a color erasing ability in the color forming system where the low-temperature color formation is effected by the low-temperature heating in the first method. Furthermore, in both the methods, two dye precursors forming different colors are separately contained in the low-temperature and high-temperature heat-sensitive layers in order to attain change in color. For example, these are proposed in JP-B-49-69, JP-B-49-4342, JP-B-49-27708, JP-A-49-86543, and JP-A-49-65239.
However, in the case of the first method, the resulting color is a mixed color and the image is indistinct, and in also the second method, the resulting image is considerably deteriorated in storage stability, and, furthermore, an intermediate layer containing the color eraser must generally be provided, which complicates the layer construction of heat-sensitive recording materials. Therefore, two-color heat-sensitive recording materials which are inexpensive and superior in quality cannot be obtained.
On the other hand, as dye precursors which individually can form multicolors, triphenylmethane compounds and indolylphthalide compounds are known in JP-B-60-25276 and "Shikizai Kyokaishi", 64 (7), 425-430 (1991). According to these techniques, two of organic acidic substances such as phenol compounds and organic carboxylic acids and organic basic substances such as guanidine compounds are contained in the same recording layer, and thus it is attempted to realize two-color heat-sensitive recording materials by these different kinds of color developers. Change of color depending on change in heating temperature becomes possible by providing great differences in melting point and dissolving speed of these two color developers. However, since the compounds having just the opposite properties, namely, an acid and a base are used as color developers, the resulting color images are not distinct and further are not utterly practical with regard to image fastness.
Furthermore, as dye precursors capable of being changed in the absorption spectrum utilizing thermal decomposition of carbamate group, U.S. Pat. No. 4,602,263 discloses various triarylmethane dyes, fluoran dyes and thiazine dyes containing thermally unstable carbamate group. However, the object of the invention is formation of full color images, and change of visual absorption spectrum in color forming state of the exemplified thermally decomposable dye precursors is limited to from colored state to colorless state or from colorless state to colored state.
As explained above, there has been known no dye precursor which changes in its chemical structure depending on the change of the heat energy applied and which singly can change clearly from one color to another color by a color forming reaction with a single color developer. Furthermore, color forming recording materials having a single recording layer which forms two colors with one kind of dye precursor have not been practically used as a two-color forming heat-sensitive recording system.